Ultrastructural evidence for a granule-containing sensory-motor-interneuron in Hydra littoralis

Abstract

An electron microscopic survey of all identifiable neural tissue in Hydra littoralis has revealed the presence of a single type of central neuron having unique features. Except for its location near the bases of epitheliomuscular cells this neuron closely resembles a peripheral sensory cell. It has a cilium, a pair of centrioles, striated rootlets, and membrane-bounded granules that appear to arise from the Golgi apparatus. Granules that are transported into the neurites may function either as neurosecretory granules or as dense-cored synaptic vesicles; those that aggregate near the cilium may function as pigment granules. The neuron has synaptic junctions with both effector cells (nematocytes and epitheliomuscular cells) and other neurons. Evidence is presented that the unique neuron is the ganglion, neurosensory, and neurosecretory cell described separately by others. It is morphologically primitive in that it has features of sensory, motor, and interneuronal cells of higher animals; therefore, I propose it be called a sensory-motor-interneuron.